1. Field of the Invention
The present invention relates to the surface treatment of substrate. More particularly, the present invention relates to a method for plasma treatment of substrate, which comprises treating the surface of a substrate with a plasma generated using a pulse-modulated high-frequency electric field; as well as to an apparatus for plasma treatment of substrate.
2. Description of the Related Art
Conventional apparatuses for microwave plasma etching have, for example, a structure of FIG. 10 disclosed in Japanese Patent Application Kokai (Laid-Open) No. 155535/1981. In this apparatus, an etching treatment for substrate is conducted by introducing a microwave of about 2.45 GHz into a plasma-generating chamber 1 of gastight structure from a microwave electric source 3 through a waveguide 2, allowing the microwave and a magnetic field of a permanent magnet or coil 6 to act on a raw material gas to generate an electron cyclotron resonance discharge plasma 7, placing a substrate 9 (a raw material to be etched) in the plasma 7, and applying a bias of several hundreds of KHz to several tens of MHz to the substrate 9.
T he above apparatus is used under continuous discharging. In using this apparatus for the purpose of the surface treatment of a substrate, there was the following problem. That is, since there is a difference in speed between electron (negatively charged) and positive ion (positively charged) as shown in FIG. 2, negative charges are accumulated on the substrate, whereby the substrate is damaged. In order to suppress this charge accumulation, surface treatment of the substrate using pulse-modulated plasma was proposed in, for example, Japanese Patent Application Kokai (Laid-open) No. 334488/1993.
In the surface treatment of a substrate using pulse-modulated plasma, charge accumulation on the substrate can be reduced by subjecting a high-frequency electric field to pulse modulation for 10 to 100 .mu.sec to reduce the electron temperature during the OFF time of the high-frequency electric field, as shown in FIG. 3. Further, high-speed etching can be expected in a halogen-based or oxygen plasma generated from chlorine, carbon tetrafluoride, sulfur hexafluoride, oxalic acid or the like because, in such a plasma, a negative ion is generated by pulse discharging, making possible etching both by positive ions and negative ions.
Furthermore, charge accumulation on the substrate can be almost completely eliminated by allowing the negative ion generated in plasma at low electron temperatures, to hit a substrate, together with a positive ion, at a low-frequency bias of 600 KHz or less. FIG. 4 shows dependency of accumulated charge on pulse OFF time in chlorine ECR plasma. Charge accumulation can be suppressed when the OFF time is 50 .mu.sec or more and the amount of negative ion generated is large. Thus, when the electron temperature and electron density are low and the plasma is constituted by positive and negative ions and, under such conditions, a low-frequency RF bias is applied to a substrate, the positive and negative ions hit the substrate alternately and charge accumulation on the substrate can be suppressed.
The above conventional techniques, however, still have a room for improvement because the electron temperature in plasma makes a sharp increase when a pulse is applied, making it difficult to keep the electron temperature sufficiently low and making impossible the thorough elimination of charge accumulation on the substrate. Further, there was a problem to be solved because the negative ion in halogen-based or oxygen plasma generated from chlorine, carbon tetrafluoride, sulfur hexafluoride, oxalic acid or the like decreases at the time of high-frequency application, allowing charge accumulation to take place easily at the time of pulse application.
FIG. 5 shows the change of electron temperature with time when a pulse is applied in a chlorine plasma. A negative ion shows overshoot in about 10 .mu.sec after the power is turned ON, under the conditions of microwave power of 500 W, and chlorine pressure of 2 mTorr, and the electron temperature reaches about 4 eV. This is brought about by the inflow of highenergy electron caused by electrons cyclotron resonance. It indicates that when a rectangular pulse is applied, there is a periodical sharp rise for about 10 .mu.sec from the moment of pulse application.
FIG. 6 shows dependency of Si etching rate on the length of pulse application time (hereinafter referred to as pulse width) when an RF bias of 600 KHz is applied to a substrate in a chlorine plasma. When the pulse width is 30 .mu.sec or more, the etching rate decreases as the pulse width increases, even if the pulse OFF time is fixed. This is because the density of negative ions decreases with an increase in electron temperature, at the time of pulse application. When the pulse width is 10 .mu.sec, the etching rate is lower than when the pulse width is 30 .mu.sec. This is because the plasma density itself is lower owing to a lower duty ratio. Therefore, it is clear that a shorter pulse ON time generates a larger amount of a negative ion but a very short pulse ON time decreases the amount.